1. Field of the Invention
The present invention relates generally to optical pickup devices and particularly to optical pickup devices having a function correcting spherical aberration.
2. Description of the Background Art
Compact disks (CDs), digital versatile disks (DVD) and other similar optical disks have their stored information reproduced via apparatuses including an optical pickup device to optically read the information from the optical disks.
In an optical pickup device a source of light emits a laser beam of light which is in turn reflected by a mirror and gathered by an objective lens to illuminate an optical disk's recording surface on a desired track. The track reflects and thus returns the light which is in turn detected by a photodetector to read the track's information.
The light gathered by the objective lens can provide spherical aberration. Spherical aberration means that, of rays focused by the objective lens, a ray passing through a portion of the lens remote from the optical axis, or the lens's peripheral portion, and a ray passing close to the optical axis, or the lens's center, are focused at different locations. When spherical aberration is introduced, an optical disk cannot have its information satisfactorily reproduced.
Accordingly, the objective lens is designed to correct spherical aberration for a thickness of an optical disk predetermined. If the optical disk is not uniform in thickness, however, and an amount of correction does not match, spherical aberration is introduced. Furthermore, if the optical disk has a recording layer formed of multiple layers, and a jump is made to a different layer, an amount of correction does not match, and spherical aberration is introduced.
Such spherical aberration is prevented for example by a method employing a liquid crystal device to modulate a phase corresponding to the spherical aberration, as disclosed in Japanese Patent Laying-Open No. 2003-141771.
Using a liquid crystal device to provide phase modulation as described in the publication, however, requires an optical pickup device to have a complicated structure and also contributes to increased cost.